1. Field of the Invention
This invention relates to packages for semiconductor devices (e.g., integrated circuit chips, discrete devicc chips, or both) and, more particularly, to such packages designed to reduce leakage, for example, of encapsulant from the package or of harmful substances (e.g., vapors, liquids, particles) from the ambient into the package.
2. Discussion of the Related Art
In the semiconductor device industry it is common to fabricate device packages from a metallic base and plastic or other polymer sidewalls. One of the many potential package types is known as an open cavity package, which is commonly used in applications where high thermal loads must be handled including, but not limited to, power devices (e.g., radio frequency, laterally diffused MOSFETs or RFLDMOSFETs). The most common open cavity package includes a high thermal conductivity base, ceramic side walls and embedded leads. These packages are typically of the hermetic or semi-hermetic variety. In both varieties the semiconductor device or chip is connected to the base and the leads, and the chip is protected from the outside environment by a substantially leak-tight sealed lid. As such, there is no requirement for semiconductor device encapsulant for environmental protection. For lower cost applications, the ceramic side walls of the package can be replaced with plastic. Many of the open cavity plastic packages are non-hermetic by design. In this case the semiconductor device must be encapsulated so that unwanted environmental degradation does not occur. The best environmental protection is achieved when the entire cavity (including the semiconductor device, wire bonds, package leads, and package base) is filled with a protective encapsulant such as silicone.
We have found that non-hermetic plastic packages fabricated in this or similar fashion have a number of problems associated with encapsulant leaking out of the cavity during the cavity fill process. Often the encapsulant leaking problem is not present in as-received open cavity packages. However, after die attach and wire bonding, when the packages are typically filled with encapsulant, we have observed that encapsulant does leak out at the plastic-to-metal interfaces of the package. This leakage results primarily from degradation of the interfaces between the different parts of the package, which in turn results from differences in the thermal expansion coefficients of the base, lead frame and sidewall materials as well as from less than ideal design. The difference in thermal expansion between the plastic side walls and the metal base and metal leads causes the plastic and metal to separate in one or more locations along the bond line. This separation provides leakage paths that allow the encapsulant to flow from the cavity to the outside surfaces of the package.
Thus, a need remains in the art for a semiconductor device package that reduces the leakage of encapsulant from the filled cavity.
In addition, these leaky interfaces also permit harmful substances (e.g., moisture, solvents, air-born particles) from the ambient to enter the package, where they can have a deleterious effect on device operation or package integrity.
Thus, a need remains in the art for a semiconductor device package that reduces the leakage of such harmful substances from the ambient into the package.